The term “functionalization” and related terminology are used in the art to refer to the process of treating a material to alter its surface properties to meet specific requirements for a particular application. For example, a surface of a material may be treated to render this surface particularly i) hydrophobic and/or ii) oleophobic and hydrophilic and/or iii) oleophilic as may be desirable for a given use. Thus, surface functionalization has become common practice in the manufacture of many materials because it adds value to the end product. In order to achieve diverse results, the surface functionalization may be carried out in a variety of ways ranging from gaseous and wet chemistry to vacuum and atmospheric plasma, as well as various vacuum chemical vapor and evaporation and sputtering deposition methods.
The processes of functionalization of porous materials or media have been accompanied with practical shortcomings caused by the processes themselves. Specifically, the degree of porosity of the materials of interest is typically reduced as a result of functionalization of the surface of the material. Non-limiting examples of porous materials include textiles, non-woven products and paper (all of which have inherent properties derived from the nature of the constituent fibers); In forming such materials, various synthetic and natural fibers (for example, polypropylene, nylon, polyethylene, polyester, cellulosic fibers, wool, silk, and other polymers and blends, to name just a few) can be shaped into different products with a great range of mechanical and physical properties, and used to create protective uniforms, biomedical fabrics and membranes, housing products, and filter media for gas and liquid filtration. The porosity of these materials usually serves a necessary function, such as, for example, the ability to be permeated with a fluid (such as gas and/or liquid), filtration of particulates, or absorption of liquids
Any treatment of a surface of a porous material carried out with the purpose of further modifying the chemical properties of the constituent fibers by appropriately functionalizing them is preferably carried out, to the extent possible, without affecting the porosity of the material. The terms nonwoven, fabric, textile, porous, and media are used interchangeably to describe a material that has a porous character.
For example, various wet-coating chemical processes have been used traditionally to treat with polymers and functionalize a fibrous material (interchangeably referred to herein as a porous substrate) that is otherwise inert or have limited surface functionality. These wet chemical processes may involve the immersion of the fibrous material in liquids or fluid foams to coat individual fibers and impart specific functionalities onto surface(s) of the fibrous material while retaining the material's porosity and its property of being permeable to fluids. Practice proves that, in spite of many claims to the contrary, such wet-chemistry processes at best materially reduce the porosity of the fibrous material at hand and, in the worst cases, essentially cause plugging of the interstices between individual constituent fibers. While the functionalization of a porous substrate by wet-chemistry may produce the desired results in terms of achieving the desired functionality of the treated surface(s), it also can cause deterioration of the mechanical characteristics of such substrate (either due to reaction with solvents used or exposure to the high temperature required to effectuate a reaction between the functional groups and the substrate, for example). Furthermore, the use of water-based chemical solutions requires the use of an energy-consuming drying oven and the use of organic-solvent-based solutions requires the use of a solvent recovery system. In all cases, in addition to the use of water, solvents, and/or thermal energy, wet surface treatment (or finishing) processes also create significant quantities of hazardous waste.
Alternative processes, such as vacuum deposition of functional polymers, have been used rather successfully to impart particular functional properties to films, foils and porous substrates without the limitations of wet-coating processes. There is a large body of literature that addresses coatings using atmospheric and vacuum plasma processes (see, for example, U.S. Pat. Nos. 507,539, 6,444,274, 6,242,054, 6,397,458, 6,562,112, 7,244,292, 6,419,871, 675,069, 7,255,291, and 7,300,859). Vacuum plasma polymerization methods have been explored for at least 40 years. Vacuum-plasma based deposition of polymers can be quite effective in coating and functionalization of porous surfaces, but that process has had limited commercial success in applications that require high speed treatment (such as web coating, for example). In a plasma coating process functional molecules in the form of a gas or vapor are introduced into the plasma and are randomly activated by ionization, forming free radicals that lead to formation of a cross-linked coating on a substrate as the activated monomer molecules impinge on it. It is well recognized that the physical and chemical properties of the resulting coating are highly dependent on process parameters such as, for example, pressure, electrode geometry and type of applied voltage (DC, AC, HFAC, Microwave). Additional practical limitation of plasma functionalization of porous substrates stems from, a relatively long exposure of the substrate to the plasma required to assure that a high enough concentration of the functional moiety is deposited on the surface of the substrate. Most methods disclosed in the literature require plasma-exposure times in the range from seconds to minutes. While such long processing times can be commercially acceptable for batch applications, they are not desired for and/or not practically applicable in roll-to-roll applications that require functionalization of webs at speeds of the order of 100 to 1000 feet per minute to create products that are both functionally and economically viable. While one process of high-speed vacuum deposition of a polymer coating that is free of the plasma polymerization limitations and that has been commercially used to functionalize porous webs was addressed by related art, such process has well-defined operational limitations. As disclosed in U.S. Pat. Nos. 7,157,117, 6,468,595, 4,954,371, for example, this process utilizes flash evaporation of a monomer material that is then condensed on a several-meter wide substrate moving at speeds in excess of 1,000 ft/min, followed by radiation curing of the condensed material with the use of electron beam or UV radiation. While a variety of monomers (such as free-radical polymerizable acrylates, cationic polymerizable epoxies and vinyl monomers, for example) can be used in this process to functionalize a surface of the substrate to impart on it a wide range of functionalities (such as, for example, hydrophobicity, oleophobicity, hydrophilicity, oleophilicity, antibacterial, color, anti-stain, metal chelating and antistatic characteristics), all these monomers not only require to be exposed to radiation to be polymerized, but also have to have a high enough molecular weight and be under a high vapor pressure to be condensed on a surface and form a liquid layer that is then converted into a polymer by radiation. Thus, this process cannot utilize monomer materials characterized by such levels molecular weight and/or vapor pressure that causes them to evaporate following a contact with the surface.
In seeking ways to functionalize surfaces with monomers that cannot be effectively condensed and cross linked using a radiation source, U.S. Pat. No. 8,840,970 disclosed another vacuum-based process of functionalization of a surface moving at a high speed that involved modification of the surface with self-assembly of specific functional monomer materials. Depending on the monomer chemistry, this process can be used to create functional surfaces with different chemical properties, including low surface energy used to repel liquids such as water and organics and high surface energy used to enhance wettability of the surface. This process utilizes an oxygen plasma to activate the substrate surface via the formation of free radicals that form covalent bonds with monomers that have acrylate, vinyl, allyl or similar double-bond chemistry. The limitation of this process is that it can be used to form a monomolecular layer on the surface but not a polymer layer.
Accordingly, there remains a need in a process configured to functionalize a surface by the deposition or formation of a thin polymer layer, without the use of wet chemistry, without the presence of a plasma during polymer formation process and without the use of heat or radiation. Solution(s) provided by this invention address the functionalization of web substrates that are processed preferably at high speed in a roll-to-roll process, and although such solutions apply to various types of substrates (including 3D objects), the main focus is made on substrates that have certain level of porosity.